Freezers for storing biological specimens, samples, materials, products and the like often use cryogenic liquids as a refrigerant. Such freezers typically feature a reservoir of a liquid cryogen, such as liquid nitrogen, in the bottom of the freezer storage chamber with the product stored above the reservoir or partly submerged with in the cryogenic liquid. The freezers typically also feature a double-walled, vacuum insulated construction so that the storage chamber is well insulated. Such freezers provide storage temperatures ranging from approximately −90° C. to −195° C.
A disadvantage of prior art liquid cryogen freezers is that the temperature cannot be directly controlled. The temperature is controlled by maintaining the amount of cryogenic liquid in the reservoir. The temperature of the freezer storage compartment thus varies dependent upon the amount of liquid cryogen in the freezer.
A further disadvantage of prior art liquid cryogen freezers is that there is some concern that submerging biological specimens in the cryogenic liquid presents a risk of cross-contamination between specimen containers. Even when the stored specimen containers are placed in the cold vapor above the cryogenic liquid reservoir, there is still the potential for the specimen containers to come into contact with, or be submerged within, the cryogenic liquid if the freezer is overfilled with the cryogenic liquid.
Also available are freezers that use mechanical refrigeration systems in place of a liquid cryogen reservoir. The mechanical refrigeration systems typically include a compressor, an evaporator, a condenser and a fan. Air is circulated through the storage chamber and across a cooling coil to maintain the desired temperature in the freezer storage chamber. The freezers normally do not feature vacuum insulation and employ materials such as foam and/or fiberglass insulation to insulate the storage chamber. Such freezers typically provide storage temperatures in the −40° C. to −80° C. range.
A disadvantage of the mechanical freezer is that the mechanical refrigeration system requires a significant amount of electrical power to maintain the desired temperature within the freezer storage chamber. Furthermore, mechanical refrigeration systems remove heat from the storage chamber and reject it to the environment around the freezer. This adds significant heat to the room within which the freezer is stored so that additional air conditioning capacity is required for the room. This adds additional electrical power requirements to the facility. In addition, in the event of a power failure, the storage chamber will warm rapidly, which could result in the loss of the stored biological materials.